Optical devices such as optical switches based on the free-carrier effect are widely used in data communications and processing. In particular, optical switches have a number of advantages over electrical switches, including switching speed, power consumption, and stability. There is also a wider range of opportunities to integrate multiple functions into an optical switch assembly, as compared with microelectromechanical (MEMS) switches, for example. Interferometer-based optical switches use optical phase shifters to achieve the required switching function.
Typically, optical devices such as optical switches and modulators are built on various platforms such as silica-on-silicon, AlGaAs/GaAs, InP, and others known in the art. Silicon-on-Insulator (SOI) platform is often seen as advantageous as it allows for a compact form factor based on the large refractive index contrast inherent in that platform.
A problem that arises with carrier-effect based optical phase shifting based devices is that a temperature difference (ΔT) may be induced in different parts of the device, as a result of self-heating due to driving part of the optical device with an electrical voltage. As a result of self-heating in the driven part, and no heating in parts of the optical device which are not driven, a ΔT results, causing the phase shift to deviate from the value that was optimized during device design. For example, in Mach-Zehnder interferometer-based optical switches using this type of phase shifter, self-heating can cause a temperature difference which induces an unexpected phase shift (namely phase error) between the second arm and the first arm of the device during the switching operation. This phase error causes a deterioration in the switch output contrast ratio.
The ΔT is problematic as the temperature coefficient of refractive index (dn/dT) for each of the arms of an optical device is typically greater than zero for an inorganic material such as silicon and silicon dioxide. As a result, a ΔT between the second arm and the first arm results in a difference in the refractive index of the first arm relative to the second arm proportionate to ΔT and dn/dT.
A difference in the refractive index induced by self-heating between the first arm and the second arm results in a time-varying extra phase shift from the desired phase shift between the two arms, which may increase the cross talk of an optical switch or induce an error in a phase shifter. As a result, carrier effect based optical devices are commonly prone to optical phase drift, which arises from self-heating of the first arm of the optical device during operation. An external compensation circuit could be used as a solution to compensate the phase drift caused by self-heating. However, this complicates the required control circuit, which has to take into account the time dynamics of self-heating.
Therefore, there is a need for an optical device that is resistant to driving current induced crosstalk, and other limitations of the prior art.
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